Wireless automatic meter reading systems are well known. Typically, each utility meter is provided with a battery-powered encoder that collects meter readings and periodically transmits those readings over a wireless network to a central station. The power limitations imposed by the need for the encoder to be battery powered and by regulations governing radio transmissions effectively prevent direct radio transmissions to the central station. Instead, wireless meter reading systems typically utilize a layered network of overlapping intermediate receiving stations that receive transmissions from a group of meter encoders and forward those messages on to the next higher layer in the network as described, for example, in U.S. Pat. No. 5,056,107. These types of layered wireless transmission networks allow for the use of lower power, unlicensed wireless transmitters in the thousands of end point encoder transmitters that must be deployed as part of a utility meter reading system for a large metropolitan area.
In 1985, as an attempt to stimulate the production and use of wireless network products, the FCC modified Part 15 of the radio spectrum regulation, which governs unlicensed devices. The modification authorized wireless network products to operate in the industrial, scientific, and medical (ISM) bands using spread spectrum modulation. The ISM frequencies that may be used include 902 to 928 MHz, 2.4 to 2.4835 GHz, and 5.725 to 5.850 GHz. The FCC allows users to operate spread spectrum wireless products, such as utility metering systems, without obtaining FCC licenses if the products meet certain requirements. This deregulation of the frequency spectrum eliminates the need for the user organizations to perform cost and time-consuming frequency planning to coordinate radio installations that will avoid interference with existing radio systems.
Spread spectrum modulators use one of two methods to spread the signal over a wider area. The first method is that of direct sequence spread spectrum, or DSSS, while the second is frequency hopping spread spectrum, or FHSS. DSSS combines a data signal at the sending station with a higher data rate bit sequence, which many refer to as a chipping code (also known as a processing gain). A high processing gain increases the signals resistance to interference. FHSS, on the other hand, relies on the distribution of a data signal randomly hopped across a number of defined frequency channels to avoid interference.
FHSS operates by taking the data signal and modulating it with a carrier signal that hops from frequency to frequency as a function of time over a wide band of frequencies. With FHSS, the carrier frequency changes periodically. The frequency hopping technique reduces interference because an interfering signal from a narrowband system will only affect the spread spectrum signal if both are transmitting at the same frequency and at the same time. Thus, the aggregate interference will be very low, resulting in little or no bit errors.
A hopping code determines the frequencies the FHSS transmitter will transmit and in which order. To properly receive the signal, the FHSS receiver conventionally is set to the same hopping code and listens to the incoming signal at the right time and correct frequency. In order for this approach to be effective, however, both the FHSS transmitter and FHSS receiver must be synchronized with one another on the same hopping code pattern and must be tracking on the same frequency.
Synchronization can be accomplished by synchronizing the FHSS transmitter and receiver in time as described, for example, in U.S. Pat. No. 5,386,435, but this requires either extremely accurate clocks in both the FHSS transmitter and receiver or some external channel that is used to synchronize the clocks. More conventionally, an encoded preamble at the beginning of each transmission is used to synchronize the FHSS transmitter and receiver. U.S. Pat. No. 6,052,406 describes a FHSS system that utilizes a correlator to synchronize an incoming sampled data stream with a known sync pattern once a phasing arrangement partitions the sampled data stream into a first and second sampled sequences. U.S. Pat. No. 6,052,407 describes a FHSS system for a cordless telephone system that builds up a table of the spectrum energy of transmissions over time and uses this table to correlate further incoming signals to determine synchronization to the frequency hopping pattern. U.S. Pat. No. 6,178,193 describes an arrangement the uses a correlated power calculation for fading periods to adjust the transmission power level of a FHSS transmitter to achieve better synchronization.
Tracking of FHSS transmissions has conventionally relied on the stability of the transmitted frequency. Generally, a transmitter will wander or drift in frequency over time due to aging or changes in temperature or voltage. Frequency stabilization circuitry has been traditionally incorporated at the FHSS transmitter level in order to control and adjust for any frequency drifting. Synthesizers, such as a phased lock loop (PLL), are used to control or stabilize the transmitter's output frequency as described, for example, in U.S. Pat. No. 5,940,428. Each modulated signal passes through this circuitry before transmission. Unfortunately, such PLL circuitry causes an unwanted drain on power and adds significant costs to the FHSS transmitter. In a wireless meter reading system, for example, where cost and battery power are central concerns, these undesirable consequences of stabilization circuitry can erect a significant manufacturing and system design barrier.
It is possible to eliminate the synthesizer circuitry at the FHSS transmitter level. Conventional technology adjusts for this frequency wandering of the transmitter signal by increasing the FHSS receiver's intermediate frequency (IF) bandwidth to accommodate for the frequency drifting. However, this solution decreases the FHSS receiver sensitivity as the FHSS receiver IF bandwidth increases. In low power transmissions for a FHSS system, high receiver sensitivity is essential in order to be able to pick out weak FHSS signals from background noise.
U.S. Pat. No. 6,188,715 describes a FHSS system for multiple sensor transmitters that intermittently transmit very short status messages. The FHSS receiver utilizes a Fast Fourier Transform (FFT) to detect transmitted carrier power at several different frequencies in order to improve synchronization and signal acquisition. Once the FFT determines which frequency contains a signal of interest from the wideband FHSS signal, the FHSS receiver tunes one or more narrowband frequency receivers using a digitally programmable finite impulse response (FIR) filter in response to the output of the FFT and the status of time and frequency registers representing the frequency hopping code. While this arrangement can improve synchronization, the use of separate narrowband frequency receivers still requires that the FHSS transmitters utilize frequency stabilization circuitry in order to provide sufficient frequency tracking.
As a result of these undesirable barriers associated with the use of frequency stabilization circuitry in conventional meter reading systems, there exists a need for a low cost, low power, receiver that is capable of identifying, locating, and tracking FHSS signals received from a transmitter that does not utilize frequency stabilizing circuitry. Additionally, the receiver must be able to accommodate for these potentially unstable signals while at the same time maintaining high receiver sensitivity.